1. Field of the Invention
The present invention relates generally to a pressure control valve for an active suspension control system of an automotive vehicle. More specifically, the invention relates to pressure control valve arrangements which include an electromagnetic proportional pressure reducing valve as a pilot valve and a pressure reducing valve as a main valve which is activated by a pilot pressure output from the pilot valve.
2. Description of the Background Art
Japanese Utility Model Publication Nos. 1-116813 and 61-102972 assigned to the owner of this application disclose a typical pressure control valve for an active suspension system. The pressure control valve of the Publication No. 1-116813 includes generally a three-way spool valve as a main valve and an electromagnetic proportional relief valve type pilot valve. The main valve comprises a supply port connected to a hydraulic power source which incorporates a pump and a reservoir tank, a return port, and an output port connected to a suspension actuator. The pilot valve is operable to provide a pilot pressure to an end of a spool of the main valve. To the other end of the spool, a controlled pressure from the output port is fed back for adjusting the controlled pressure dependent upon the pilot pressure.
In the pressure control valve of the Publication No. 61-102972, a controlled pressure is fed back to an end of a spool of a three-way spool valve, while a pilot pressure is supplied to the other end of the spool, thereby producing the controlled pressure of a pressure level determined by the pilot pressure which varies in proportion to thrust of a proportional solenoid. The pilot pressure is provided by a pressure reducing valve which blocks flow input when providing no pilot pressure.
However, the prior art pressure control valve of the Publication No. 1-116813 is arranged such that a hydraulic fluid always flows in the pilot valve, resulting in great consumption of a useless pilot flow rate. Especially, when a control range is made wider, or a supply pressure is elevated to a high level, the useless consumption of the pilot flow rate is extremely increased. Therefore, energy efficiency is degraded and fuel consumption of a vehicle is increased.
Further, the pressure control valve of the Publication No. 61-102972 is arranged such that the main valve directs a supply pressure to the pilot valve. When a supply flow rate to the pilot valve is increased for enhancing response of the pilot valve, a controlled pressure output from the main valve tends to vary due to vibration input from a road surface. This results in unstable control of the pilot valve. Therefore, achievement of response and stability of system control is practically difficult.
For avoiding the above disadvantages, it may be proposed that a supply pressure from a hydraulic power source is modified by an electromagnetic three-way spool valve to provide a pilot pressure. However, in such arrangements, response when increasing a controlled pressure is different from that when decreasing it. Another problem occurs in that regulation of a controlled pressure is difficult.